The present invention relates to a high pressure metal vapor discharge lamp, and more particularly to an improved discharge lamp utilizing a radioactive substance in the discharge tube.
Generally, high pressure metal vapor discharge lamps, such as metal halide lamps and high pressure sodium vapor lamps, have high starting voltages although the luminous output and luminous efficiency thereof are high. Consequently, these lamps can only be started by applying sufficiently high voltages.
To reduce the starting voltage of such lamps, the inventors of the present invention have proposed the inclusion of a radioactive substance in the discharge tube of the lamps in their U.S. patent application Ser. No. 265,675. In this proposal, seeds or triggers for initial discharges are provided by electrons supplied from a radioactive substance having a short half-life ranging from 0.5 to 10.sup.4 years contained in the discharge tube of the lamp.
However, the radioactive substance having a short half-life is hazardous to humans, thus requiring extreme care and duplicate or triplicate security measures in its handling. Since various injuries are caused when the radioactive substance comes in contact with a human body, countermeasures must be taken to prevent the radioactive substance from being spattered in nearby areas.
Ordinarily, it is required that the radioactive substance be subjected to a smear test to establish that the substance may be safely handled without separating any trace of the substance. In the smear test, a radioactive source is contacted with a sheet of filter paper, and the paper sheet is then tested with a radioactive counter for the presence of any radioactivity. In other words, the smear test determines whether or not the radioactive substance tends to be displaced from the source to the filter paper.
U.S. patent application Ser. No. 273,112 discloses a countermeasure technique for preventing the spread of radioactive material which comprises dispersing or impregnating a radioactive substance in a ceramic material so that it will pass the smear test. However, the requirement for safety is limitless, and further improvement thereof is always desirable.